1. Field of The Invention
The invention relates to a screening method for multicolor printing wherein raster elements are produced point or line-wise in chronological succession by engraving, exposure or electro-mechanically by means of high energy radiation or electro-optically wherein the types of raster utilized in both the line spacings in the various rasters of a set-up color separation have ratios of approximately small rational numbers and the raster point spacings within the lines of the set of rasters have ratios of approximately small rational numbers and wherein the number of raster points per surface area can be different in the rasters for the different individual colors.
2. Description of the Prior Art
In the art of electronic mechanical engraving of printing forms as, for example, in the engraving of gravure cylinders it has been described in detail in "Der Polygraph", No. 18/1965; "Deutscher Drucker", No. 22-24/1974 and "Druck-Print", 12/68, pp. 931 through 938, the problem arises that in order to avoid Moire effects, an angling between the screens of the individual colors of a multi-color printing used in standard conventionally etched autotype forms cannot be accomplished.
Primarily this is because the raster grid of an engraving for example of a gravure cylinder is formed by means of successively engraving all of the raster points such that the engraving head operating with a constant rastering frequency of its engraving tool engraves one axial extending line of the cylinder after another with raster cups of different depths but with constant center spacings from each other. Thus, the engraving head continuously moves slowly or step-by-step in the axial direction of the cylinder on the respective axial line spacing per cylinder revolution which is prescribed for the desired raster.
Thus, the raster grid results from the interaction between the rotary motion of the drum, the forward feed of the engraving tool in the axial direction of the cylinder and the rastering frequency of the engraving tool. It is obvious that the geometry of the raster grid is based on the circumferential spacing and the forward feed direction as specified directions and that an angled position of the raster grid relative to the circumferential direction which is the subsequent direction of the run of paper during printing cannot occur. Since the number of cups on a given circumference as well as the length unit in the axial direction is extremely precise, Moire phenomena cannot occur even given non-angled rasters because Moire patterns result because raster points of two or more rasters are printed on top of one another or next to one another at periodically repeating intervals.
In practice, however, a number of meters or yards of the paper which are to be printed may be present between two printing units of a printing machine and the paper can easily change its length by one-half of a raster point spacing due to stretching and shrinking due to moistening and dilatation between the printing units. As a result, the raster points of two or more colors may be accidentally printed on top of each other or be printed so close next to each other to obtain so-called color drift and these effects also cause Moire effects which are statistically much greater so that impressions of the same subject printed from various cylinder impressions have great color fluctuations which cannot in practice be accepted.
It should also be noted that the standard angling of the rasters utilized in conventional practice does not cause the Moire to disappear. On the contrary, as a result of angling, the Moire is made so fine that its mesh is only slightly greater than the mesh of the raster itself. Thus, due to the finite power of resolution of the human eye at a reading distance, it will no longer be perceptible if a sufficiently fine screen is used.
Since the existence of electro-mechanical engraving particularly of gravure form cylinders, it has become desirable to obtain raster geometries to produce forms which are equivalent in effect to conventional angling.
A type of raster distortion in the engraving using a suitable selection of the raster production parameters has been disclosed in German LP No. 1,112,407 (also in "Druck-Print" 12/68, pp. 931-938 also). The same effect as in "angling" is created by means of "crushing" or "elongating" the originally quadratic raster meshes in specific numerical. However, it was observed at the time of this work that there are only two practical combinations of raster geometries which meet the conditions desired which are that in the circumferential direction that three raster points of the second raster occur for two raster points of the first raster and vice-versa in the forward-feed direction that three raster points of the first raster occur for two raster points of the second raster. Compare FIG. 2b in German LP No. 1,112,407. In four-color printing, all of the combinations produce an unacceptable Moire effect.